Dielectrophersis Control of InAs based Nanowires for bio sensing

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Semiconductor devices are increasingly being utilized in healthcare based sensor applications due their excellent ability to detect minute changes in their surroundings due via alterations in their electrical and optical properties. Recently there has been much interest in semiconductor nanowires, these wires are grown vertically upwards from a semiconductor surface, with nanowires having been demonstrated from a range of compound semiconductors including, Gallium Arsenide, Gallium Phosphide, Indium Arsenide and Indium Antimony. Growth of these wires is initiated either by initially depositing tiny metal droplets on the surface or by forming nanoscale holes in an oxide mask. The resultant nanowires can have lengths in excess of 1um and diameters below 100nm.These nanowires are of particular interest for bio-sensing and healthcare applications for a number of reasons, firstly their small size makes them ideal to integrate molecules and other optical and electrical components. Secondly due to the cylindrical shape of the wires the majority of the charge resides on the surface making them especially sensitive to changes in their environment, for example arising from the presence of differing bio-markers. However the vertical nature of the growth and resultant device geometry severely limits the exploitation of these nanowires as the nanowires are routinely planarised and encapsulated under a polymer layer. This results in a relatively large final device and also supresses much of the surface charge. The proposed work here offers a step-change in the development of nanowire based sensors and devices by utilizing the process of dielectrophoresis. This is a process whereby the application of a non-uniform electric field results in a force upon a charged particle. By appropriately controlling the electric field it is possible to utilize this process to accurately move and position particles. To achieve this the nanowires will be removed from their native growth substrate and then dispersed in solvent before undergoing dielectrophoresis to position the wires at pre determined positions. This offers an attractive route to undertake nanowire bio-sensing and monitoring due to the ability to integrate the nanowires directly with microfluidics and other optical or electrical systems thus exploiting the accurate and reliable positioning offered by this technique.A further advantage is that nanowire devices formed via this technique can be achieved in a horizontal rather than vertical configuration. This will make integration with other components much more straightforward and also enable the whole surface of the nanowire to interact with its environment, greatly enhancing the sensitivity of the wires to any changes in their environment. These potential benefits offer the possibility to greatly enhance the performance of nanowire based sensors and devices as well as enabling to be fabricated more efficiently and at a reduced cost.Specifically this project aims to fabricate nanowire devices via dielectrophoresis, establishing relationships between the process parameters and the final devices electrical, optical and physical characteristics, opening a route for predictable device fabrication. Finally nanowire devices will be integrated with microfluidic systems to demonstrate an ionic flow sensor, allowing both the concentration and flow rate of liquids to be monitored in a compact lab on a chip device.